1. Field of the Invention
The present invention relates to a lithium secondary battery.
2. Description of Related Art
Lithium secondary batteries have high energy densities and are widely used in notebook computers, and mobile phones, etc., taking advantage of the characteristics. In recent years, electric vehicles draw attentions from the viewpoint of preventing a global warming with an increase in carbon dioxide, and therefore it is also considered to apply the lithium secondary batteries as their power sources.
Although the lithium secondary batteries have such excellent characteristics, there are challenges with them. One of the challenges is to enhance the safety thereof, and among them, it is an important challenge to secure the safety when overcharged.
When a lithium battery is overcharged, the thermal stability thereof is decreased, thereby possibly causing the safety thereof to be decreased. Accordingly, current lithium secondary batteries have a control circuit for stopping charge to secure the safety thereof when an overcharged state is detected. The overcharged state is detected by monitoring a battery voltage. However, the difference between the operating voltage of a battery and the voltage at an overcharged state is small, and hence it has been difficult to properly detect overcharge with the control circuit. In addition, there is the possibility that overcharge may be caused if a failure occurs in the control circuit, and accordingly it is important to secure the safety of the lithium secondary battery itself when overcharged.
A technique that a phosphate is added in a battery to enhance the safety thereof is disclosed in Japanese Patent Application Laid-Open No. 2009-295507.
A technique that a phosphazene compound is added in a battery to enhance the safety thereof is disclosed in Japanese Patent Application Laid-Open No. 2005-116424.
In addition, a technique that a phosphate or a phosphazene compound is added in a battery is disclosed in Japanese Patent Application Laid-Open No. 2007-115583.